Short range wireless systems typically involve devices that have a communications range of one hundred meters or less. To provide communications over long distances, these short range systems often interface with other networks. For example, short range networks may interface with cellular networks, wireline telecommunications networks, and the Internet.
Wireless piconets, also referred to as personal area networks (PANs) typically operate in unlicensed portions of the radio spectrum, usually either in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band or the 5 GHz Unlicensed-National Information Infrastructure (U-NII) band. Examples of wireless piconet technology include the Bluetooth standard and the IEEE 802.15.3 standard.
Bluetooth defines a short-range radio network, originally intended as a cable replacement. It can be used to create ad hoc networks of up to eight devices, where one device is referred to as a master device. The other devices are referred to as slave devices. The slave devices can communicate with the master device and with each other via the master device. The Bluetooth Special Interest Group, Specification Of The Bluetooth System, Volumes 1 and 2, Core and Profiles: Version 1.1, Feb. 22, 2001, describes the principles of Bluetooth device operation and communication protocols. Bluetooth devices operate in the 2.4 GHz radio band reserved for general use by Industrial, Scientific, and Medical (ISM) applications. These devices are designed to find other Bluetooth devices within their communications range and to discover what services they offer.
IEEE 802.15.3 defines a framework for devices to communicate at high data rates (e.g., 55 Mbps) at short ranges across ad hoc networks. Currently, an IEEE 802.15.3 piconet may support a large number of devices, such as 250. These devices share frequency channels by employing time division multiple access (TDMA) transmission and Carrier Sensing Multiple Access (CSMA) techniques. IEEE 802.15.3 piconets include a device known as a piconet controller or coordinator (PNC) and one or more other devices (referred to as DEVs).
The PNC is a device that controls piconet resources. In particular, the PNC performs functions, such as controlling the basic timing for the piconet, and regulating the admission of devices into the piconet. In addition, the PNC manages quality of service (QoS) and security aspects of the piconet. To perform these functions, the PNC typically cannot enter an “idle” or “sleep” mode. Thus, the PNC consumes more power than the other devices in the piconet. Therefore, it is desirable to assign the PNC role to devices having a good battery condition, or even a fixed power supply.
Multiple devices may join and leave the piconet during its existence. Likewise, different devices may assume the PNC role at various times. The process in which the PNC role is transferred between a first device and a second device is referred to herein as PNC handover. IEEE 802.15.3 provides for PNC handover through the use of a PNC handover command, which is issued for various reasons. However, these reasons do not currently include the status of a battery, but only information on the availability of a fixed power supply. In an ad hoc network, none of the devices may have a fixed power supply. The knowledge of battery levels in such a network could be used for balancing power consumption. The inability to determine the battery level in such a network may also cause a device with low battery power (that otherwise appears as a good PNC candidate) to be unable to reject the PNC role because of its low battery power, and run out of power after the PNC role is transferred to it. Accordingly, techniques are needed for taking available power source capacity of devices into consideration during PNC handover processes.
In addition to the short-range networking techniques described above, ultra wideband (UWB) techniques have become an attractive solution for short-range wireless communications because they allow for devices to exchange information at relatively high data rates. Current FCC regulations permit UWB transmissions for communications purposes in the frequency band between 3.1 and 10.6 GHz. However, for such transmissions, the spectral density has to be under −41.3 dBm/MHz and the utilized bandwidth has to be higher than 500 MHz.
There are many UWB transmission techniques that can fulfill these requirements. A common and practical UWB technique is called impulse radio (IR). In IR, data is transmitted by employing short baseband pulses that are separated in time by gaps. Thus, IR does not use a carrier signal. These gaps make IR much more immune to multipath propagation problems than conventional continuous wave radios. RF gating is a particular type of IR in which the impulse is a gated RF pulse. This gated pulse is a sine wave masked in the time domain with a certain pulse shape.
IR transmission facilitates a relatively simple transmitter design, which basically requires a pulse generator and an antenna. This design does not necessarily require a power amplifier, because transmission power requirements are low. In addition, this design does not generally require modulation components such as voltage controlled oscillators (VCOs) and mixers, because the impulses are baseband signals.
In general, IR receiver designs are more complex than their corresponding transmitter designs. However, these designs are much simpler than conventional receiver designs because they typically do not employ intermediate frequency (IF) signals or filters. However, to fulfill spectral requirements, IR impulses have to be very short in duration (e.g., a couple of nanoseconds). This requirement places stringent timing demands on receiver timing accuracy. The fulfillment of these demands can also provide IR receivers with accurate time resolution and positioning capabilities.